Pixel formation method

ABSTRACT

A method for forming a pixel includes forming, in a semiconductor substrate, a wide trench having an upper depth with respect to a planar top surface of the semiconductor substrate. The method also includes ion-implanting a floating-diffusion region between the planar top surface and a junction depth in the semiconductor substrate. In a cross-sectional plane perpendicular to the planar top surface, the floating-diffusion region has (i) an upper width between the planar top surface and the upper depth, and (ii) between the upper depth and the junction depth, a lower width that exceeds the upper width. Part of the floating-diffusion region is beneath the wide trench and between the upper depth and the junction depth.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.16/711,239 filed Dec. 11, 2019, the entire content of which isincorporated herein by reference.

BACKGROUND

Camera modules in commercial products such as stand-alone digitalcameras, mobile devices, automotive components, and medical devicesinclude an image sensor and a pixel array thereof. The pixel arrayincludes a plurality of pixels. A pixel array's pixel density is thenumber of pixels per unit area on the image sensor. In operation, thelens of a camera module forms an image, on the image sensor, of anobject in its field of view. The object can be viewed a plurality ofinfinitesimally small point-sources of illumination—“impulses”—incidenton the camera. The lens images each of the plurality of impulses at aplane of the pixel array as a respective one of a plurality ofpoint-spread functions—“impulse responses.” The resolution of imagescaptured by the image sensor depends in part on pixel size compared tothe size of the impulse response. Accordingly, one way to increase acamera's maximum attainable resolution is to increase pixel density bydecreasing pixel size. Motivation to decrease pixel sizes has led todevelopment of pixels with vertical transfer gates.

Each pixel of the plurality of pixels includes a photodiode region, afloating diffusion region, and a transfer gate. The transfer gatecontrols current flow from the photodiode region to the floatingdiffusion region and may be part of a field-effect transistor. Theelectric potential of the photodiode region exceeds that of the floatingdiffusion region. Light reaching the photodiode region generatesphotoelectrons. Turning on the transfer gate forms a conducting channelthat allows the accumulated photoelectrons to transfer or flow fromphotodiode region to the floating diffusion region. When the transfergate is pulsed to an off-state, the potential barrier is higher thanthat of the photodiode region, hence preventing photoelectrons fromflowing to the floating diffusion region.

In one common pixel architecture, the photodiode and the floatingdiffusion region are laterally displaced within the pixel, in a lateraldirection parallel to a plane of the pixel array, with the transfer gatetherebetween. This plane is horizontally orientated with respect to thevertical direction perpendicular thereto that defines the direction ofnormally-incident reaching the pixel array. Such a horizontalorientation limits how much the pixel density can be decreased. Hence,one way to increase pixel density is to orient the photodiode, transfergate, and floating diffusion in a direction that has a verticalcomponent. Such transfer gates are examples of vertical transfer gates.

SUMMARY OF THE EMBODIMENTS

While vertical transfer gates enable increased pixel density, pixelswith vertical transfer gates are vulnerable to an image artifact knownas blooming. Blooming occurs when photogenerated charge in pixelsaturate the pixel by exceeding the pixel's full-well capacity, andblooms over to adjacent pixels. Excess photoelectrons generated by thesaturated pixel are collected by adjacent pixels, which affectssensitivity of the adjacent pixels. Blooming increases as the distancebetween the pixel's photodiode and its floating diffusion regionincreases, for example, when this distance exceeds a distance to aneighboring pixel. Embodiments disclosed herein ameliorate this problem.

In a first aspect, a pixel includes a semiconductor substrate, afloating diffusion region, and a photodiode region. The semiconductorsubstrate has a substrate upper surface forming a trench extending intothe semiconductor substrate. The trench has a (i) trench depth relativeto a planar region of the substrate upper surface surrounding thetrench, and in a cross-sectional plane perpendicular to the planarregion and in a direction parallel to the planar region (ii) an upperwidth between the planar region and an upper depth that is less than thetrench depth, and (iii) a lower width, between the upper depth and thetrench depth, that is less than the upper width. The floating diffusionregion is in the semiconductor substrate, adjacent to the trench andextends away from the planar region to a junction depth, relative to theplanar region, that exceeds the upper depth and is less than the trenchdepth. The photodiode region is in the semiconductor substrate andincludes (i) a lower photodiode section beneath the trench and (ii) anupper photodiode section adjacent to the trench, beginning at aphotodiode depth that is less than the trench depth, extending towardand adjoining the lower photodiode section.

In a second aspect, a method for forming a pixel includes forming a widetrench in a semiconductor substrate and ion-implanting afloating-diffusion region in the semiconductor substrate between theplanar top surface and a junction depth in the semiconductor substrate.The wide trench has an upper depth with respect to a planar top surfaceof the semiconductor substrate. The floating-diffusion region has, in across-sectional plane perpendicular to the planar top surface, (i) anupper width between the planar top surface and the upper depth, and (ii)between the upper depth and the junction depth, a lower width thatexceeds the upper width. Part of the floating-diffusion region isbeneath the wide trench and between the upper depth and the junctiondepth.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 depicts a camera imaging a scene.

FIG. 2A is a cross-sectional schematic of a semiconductor substrate,which is an embodiment of the semiconductor substrate of the camera ofFIG. 1.

FIG. 2B is a circuit diagram of a four-transistor (“4T”) pixel, which isan example pixel circuitry architecture of a pixel of FIG. 2A.

FIG. 3 is a cross-sectional schematic of a pixel, which is an example ofa pixel formed in the semiconductor substrate of FIG. 2, in anembodiment.

FIG. 4 is a cross-sectional view of the pixel of FIG. 3 with theaddition of dielectric layer, in an embodiment.

FIGS. 5 and 6 are respective schematic cross-sectional views of part ofan image sensor that includes the pixel of FIG. 4, in an embodiment.

FIG. 7 is a cross-sectional view of an image sensor, which is an exampleof the image sensor of FIGS. 5 and 6, in an embodiment.

FIG. 8 is a cross-sectional schematic of a semiconductor substrate thatincludes shallow trenches, in an embodiment.

FIG. 9 is a cross-sectional schematic of the semiconductor substrate ofFIG. 8, after formation of a floating diffusion region therein, in anembodiment.

FIG. 10 is across-sectional schematic of the semiconductor substrate ofFIG. 9 after formation of a deep trench therein, in an embodiment.

FIG. 11 is a cross-sectional schematic of a pixel, which is thesemiconductor substrate of FIG. 10 after filling its deep trench with agate-electrode material, in an embodiment.

FIG. 12 is a flowchart illustrating a method for fabricating a pixel, inan embodiment.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Reference throughout this specification to “one example” or “oneembodiment” means that a particular feature, structure, orcharacteristic described in connection with the example is included inat least one example of the present invention. Thus, the appearances ofthe phrases “in one example” or “in one embodiment” in various placesthroughout this specification are not necessarily all referring to thesame example. Furthermore, the particular features, structures, orcharacteristics may be combined in any suitable manner in one or moreexamples.

Spatially relative terms, such as “beneath,” “below,” “lower,” “under,”“above,” “upper,” and the like, may be used herein for ease ofdescription to describe one element or feature's relationship to anotherelement(s) or feature(s) as illustrated in the figures. It will beunderstood that the spatially relative terms are intended to encompassdifferent orientations of the device in use or operation in addition tothe orientation depicted in the figures. For example, if the device inthe figures is turned over, elements described as “below” or “beneath”or “under” other elements or features would then be oriented “above” theother elements or features. Thus, the terms “below” and “under” mayencompass both an orientation of above and below. The device may beotherwise oriented (rotated ninety degrees or at other orientations) andthe spatially relative descriptors used herein interpreted accordingly.In addition, it will also be understood that when a layer is referred toas being “between” two layers, it can be the only layer between the twolayers, or one or more intervening layers may also be present.

The term semiconductor substrate may refer to substrates formed usingsemiconductors such as silicon, silicon-germanium, germanium, galliumarsenide, and combinations thereof. The term semiconductor substrate mayalso refer to a substrate, formed of one or more semiconductors,subjected to previous process steps that form regions and/or junctionsin the substrate. A semiconductor substrate may also include variousfeatures, such as doped and undoped semiconductors, epitaxial layers ofsilicon, and other semiconductor structures formed upon the substrate.

Throughout this specification, several terms of art are used. Theseterms are to take on their ordinary meaning in the art from which theycome, unless specifically defined herein or the context of their usewould clearly suggest otherwise. It should be noted that element namesand symbols may be used interchangeably through this document (e.g., Sivs. silicon); however, both have identical meaning.

FIG. 1 depicts a camera 190 imaging a scene. Camera 190 includes animage sensor 100, which includes a semiconductor substrate 110.Constituent elements of semiconductor substrate 110 may include at leastone of silicon and germanium. Semiconductor substrate 110 includes apixel array 112A. Image sensor 100 may part of a chip-scale package or achip-on-board package.

FIG. 2A is a cross-sectional schematic of a semiconductor substrate 210,which is an example of semiconductor substrate 110 of image sensor 100.The cross-section illustrated in FIG. 2A is parallel to a plane,hereinafter the x-z plane, formed by orthogonal directions 298X and298Z, which are each orthogonal to direction 298Y. Herein, the x-y planeis formed by orthogonal directions 298X and 298Y, and planes parallel tothe x-y plane are referred to as transverse planes. Unless otherwisespecified, heights of objects herein refer to the object's extent indirection 298Z, or a direction 180° opposite thereto. Herein, referenceto an axis x, y, or z or associated direction ±x, ±y, or ±z refers todirections 298X, 298Y, and 298Z respectively. Also, herein, a horizontalplane is parallel to the x-y plane, a width refers to an objectsextension in the y direction, and vertical refers to the z direction.

Semiconductor substrate 210 has a bottom substrate surface 211 and a topsubstrate surface 219, each of which may be perpendicular to direction298Z. Herein, top substrate surface 219 may be referred to as the frontside surface of semiconductor substrate 210 and bottom substrate surface211 may be referred to as the backside surface of semiconductorsubstrate 210. Herein, top substrate surface 219 may be referred as thenon-illuminated surface of semiconductor substrate 210 and bottomsubstrate surface 211 opposite to top substrate surface 219 may bereferred to as the illuminated surface of semiconductor substrate 210.Semiconductor substrate 210 includes a plurality of pixels 212 that forma pixel array 212A, which is an example of pixel array 112A. Theplurality of pixels 212 are arranged in a plurality of rows and columnsin directions 298X and 298Y respectively. Pixel array 212A has a pixelpitch 213 in direction 298X. In direction 298Y pixel array 212A haspitch P_(y) that, in embodiments, equals pixel pitch 213. Inembodiments, pixel pitch 213 is less than 1.1 μm, for example, pixelpitch 213 may equal 0.9 μm.

FIG. 2B is a circuit diagram of a four-transistor (“4T”) pixel 290,which is a candidate pixel circuitry architecture of pixel 212. Pixel290 includes a photodiode PD, a transfer transistor TX, a resettransistor RST, a source follower transistor SF, a row-select transistorRS. Pixel 290 is electrically connected to a bitline 202 of image sensor100. FIGS. 2A and 2B are best viewed together in the followingdescription.

Each pixel 212 includes a respective photodiode region 240 of arespective photodiode PD, a vertical transfer gate 280 of a respectivetransfer transistor (e.g., transfer transistor TX), and a respectivefloating diffusion region 260. Photodiode region 240 of each pixel 212is configured to generate and accumulate charges in response to incidentlight thereon, for example entered from bottom substrate surface 211 ofsemiconductor substrate 210 (e.g., backside surface of semiconductorsubstrate 210) during an integration period of the image sensor 100.Electrical connection of photodiode region 240 to floating diffusionregion 260 depends on voltage applied to vertical transfer gate 280.Charges, e.g., photoelectrons, accumulated in photodiode region 240(e.g., source of transfer transistor TX), for example during anintegration period of image sensor 100, can be selectively transferredto floating diffusion region 260 (e.g., drain of transfer transistor TX)depending on voltage applied to vertical transfer gate 280 of thetransfer transistor (e.g., transfer transistor TX) associated with pixel212. The photodiode region 240 may be in form of various configurationsincluding pinned photodiode configuration, partially pinned photodiodeconfiguration.

Each vertical transfer gate 280 of the transfer transistor (e.g.,vertical gate portion of transfer transistor TX) is formed in arespective trench 220 formed by top substrate surface 219. Trench 220includes side surfaces 219S and a bottom surface 219B.

Photodiode region 240 of each pixel 212 has a full well capacity that isrelated to the number of photoelectrons its photodiode region 240 canhold during an integration period. As photoelectrons generated exceedthe full well capacity, pixel 212 becomes saturated and hence is notable to accommodate additional photoelectrons generated during theintegration period. It is desirable for the excess photoelectronsgenerated in pixel 212 to tunnel through semiconductor substrate 210 tothe nearest floating diffusion region 260, that is, the floatingdiffusion region associated with the pixel in which the photoelectronswere generated. However, when floating diffusion region 260 is disposedat a distance that is further away for the excess photoelectrons toreach within their lifetime or a time a photoelectron can move withoutrecombining with a hole, the excess photoelectrons tunnel to an adjacentunsaturated pixel 212 and are collected by the adjacent pixel'sphotodiode region 240. This process results in blooming and hencedegrades the quality of images generated by image sensor 100.

In embodiments, each pixel 212 is a four-transistor pixel or 4T pixeland further includes a reset transistor RST, a source followertransistor SF and a row select transistor RS. The reset transistor RSTis coupled between a power line and the floating diffusion 260 to reset(e.g., discharge or charge floating diffusion 260 region to a presetvoltage e.g., a supply voltage V_(DD)) under control of a reset signalduring a reset period. The reset transistor RST is further coupled tophotodiode region 240 of photodiode PD through the transfer transistorTX to selective reset photodiode region 240 to the preset voltage duringthe reset period. Floating diffusion region 260 is coupled to a gate ofthe source follower transistor SF. The source follower transistor iscoupled between the power line and the row select transistor RS. Thesource follower transistor SF operates to modulate the image signaloutput based on the voltage of floating diffusion region 260 received,where the image signal corresponds to the amount photoelectronsaccumulated in photodiode region 240 during the integration period atthe gate thereof. The row select transistor RS selectively couples theoutput (e.g., image signal) of the source follower transistor RS to thereadout column line (for example, bitline 202) under control of a rowselect signal.

In operation, during the integration period (also referred to as anexposure or accumulation period) of image sensor 100, photodiode region240 of photodiode PD detects or absorbs light incident on pixel 212.During the integration period, the transfer transistor TX is turned off,i.e., the vertical transfer gate 280 of the transfer transistor TXreceives a cut-off signal (e.g., a negative biasing voltage). Thephotogenerated charge accumulated in photodiode region 240 is indicativeof the amount of light incident on photodiode region 240 of photodiodePD. After the integration period, the transfer transistor TX forms aconduction channel along the vertical transfer gate structure andtransfers the photogenerated charge to floating diffusion region 260through the conduction channel upon reception of a transfer signal(e.g., a positive biasing voltage) at vertical transfer gate 280. Thesource follower transistor SF generates the image signal. The row selecttransistor RS coupled to the source follower transistor then selectivelyreads out the signal onto a column bit line for subsequent imageprocessing.

The disclosed vertical transfer gate structure may apply to any of avariety of additional or alternative types of pixel cell, e.g. afive-transistor pixel cell, or a six-transistor pixel cell and/or thelike.

FIG. 3 is a cross-sectional schematic of a pixel 300, which is anexample of pixel 212. Pixel 300 is formed in a semiconductor substrate310, which is an example of semiconductor substrate 210, A. Pixel 300includes a trench 320 and, in semiconductor substrate 310, a floatingdiffusion region 360, and a photodiode region 340. FIG. 3 denotestransverse planes 301, 302, 303, 304, 306, and 307, each of which areparallel to the x-y plane. In embodiments, planes 302 and 303 arecoplanar.

In embodiments, semiconductor substrate 310 is p-doped, photodioderegion 340 is n-doped, and floating diffusion region 360 is n⁺-doped. Inembodiments, floating diffusion region 360 has a dopant concentrationbetween 10¹⁹ and 5×10²⁰ charge carriers per cubic centimeter. Inembodiments, semiconductor substrate 310 is a silicon bulk substratedoped with p-type dopants e.g., boron, and photodiode region 340 andfloating diffusion region 360 are doped with n-type dopants e.g.,arsenic, phosphors. However, it should be appreciated that the polaritymay be reversed, for example, semiconductor substrate 310 may be n-typedoped, while photodiode region 340 and floating diffusion region 360 arep-type doped regions in the semiconductor substrate 310.

Semiconductor substrate 310 has a substrate upper surface 319 and asubstrate bottom surface 311 thereopposite. Substrate upper surface 319forms trench 320, which extends into semiconductor substrate 310 towardbottom surface 311 and has a trench depth 323 relative to a planarregion 318 of substrate upper surface 319 surrounding the trench. Planarregion 318 is in plane 301. Trench depth 323 defines the distancebetween transverse planes 301 and 306.

In the x-z plane and in direction 298X, trench 320 has an upper width322 between plane 301 and a shallow-trench depth 324 that is less thantrench depth 323. Shallow-trench depth 324 defines the distance betweenplanes 301 and 302. Also in the x-z plane and in direction 298X, trench320 has a lower width 327, between transverse planes 302 and 306 that isless than upper width 322. In embodiments, upper width 322 exceeds lowerwidth by two times a width 325 denoted in FIG. 3, in which trench 320 issymmetric about a plane that is parallel to the y-z plane and bisectstrench 320. In embodiments, trench 320 has the same dimensions in they-z plane as it does in the x-z plane shown in FIG. 3. In embodiments,upper width 322 is determined by width 325 and lower width 327.

Trench 320 may be viewed as including two trenches: a wide trench 321between planes 301 and 302, and a narrow trench 326 between planes 302and 306. Wide trench 321 has width 322 and shallow-trench depth 324.Narrow trench 326 that has width 327 and a depth 329, which isshallow-trench depth 324 subtracted from trench depth 323.

Floating diffusion region 360 is disposed adjacent to trench 320, andextends away from planar region 318 to a junction depth 363. Junctiondepth 363 defines the distance between transverse planes 301 and 304,exceeds shallow-trench depth 324, and is less than the trench depth 323.In embodiments, junction depth 363 exceeds shallow-trench depth 324 by adistance 328, which is between ten nanometers and one hundrednanometers, a benefit of which is reduction in junction capacitance.However, when distance 328 is less than ten nanometers, the resistancebetween the conduction channel formed by the transfer transistor TX andfloating diffusion region 360 increases, and hence impedes chargetransfer. That is, photogenerated charge accumulated in photodioderegion 340 may encounter barrier to transfer to the floating diffusionregion 360 causing continuity issues resulting in image lag. Further, arisk of distance 328 exceeding one hundred nanometers is degradedconversion gain. Increasing distance 328 increases junction area betweenfloating diffusion region 360 and the vertical transfer gate 480, whichin turn increases junction capacitance, and thus degrade conversiongain.

Photodiode region 340 is an example of photodiode region 240, FIG. 2,and includes a lower photodiode section 341 beneath trench 320 and anupper photodiode section 345 adjacent to trench 320. Upper photodiodesection 345 is formed at a photodiode depth 343, with respect to planarregion 318, that is less than the trench depth 323, and extends towardbottom substrate surface 311 to transverse plane 307. Part of upperphotodiode section 345 of photodiode region 340 is adjacent to narrowtrench 326 of trench 320. Lower photodiode section 341 adjoins upperphotodiode section 345 at horizontal plane 307 and extends away fromplanar region 318 toward bottom substrate surface 311. Lower photodiodesection 341 has a width 344 that exceeds a width 349 of upper photodiodesection 345.

Floating diffusion region 360 has an upper width 362 between transverseplanes 301 and 303 and a lower width 361 between transverse planes 303and 304. In embodiments, lower width 361 exceeds upper width 362.Substrate upper surface 319 includes a surface region 317 that is eithercoplanar with, or intersects, plane 302. Part of floating diffusionregion 360 between planes 303 and 304 is between surface region 317 andupper photodiode section 345. In embodiments, floating diffusion region360 formed directly above photodiode region 340 provides a blooming paththerebetween such that excess photoelectrons generated in photodioderegion 340 during an integration period are able to travel to floatingdiffusion region 360 within their lifetime, thus preventing blooming.

FIG. 3 denotes a thickness 312 in direction 298X of semiconductorsubstrate 310 between floating diffusion region 360 and substrate uppersurface 319. Thickness 312 may range from zero to twenty nanometers. Inembodiments, thickness 312 equals zero such that substrate upper surface319 between planes 301 and 304 is a surface of floating diffusion region360. A risk of thickness 312 exceeding twenty nanometers is increasedjunction capacitance, which degrades conversion gain.

In embodiments, a passivation implant region may be disposed in thespace between floating diffusion region 360 and substrate upper surface319, i.e., between floating diffusion region 360 and shallow trench withdielectric layer 450 (FIG. 4) or thickness 312, to passivate substrateupper surface 319 of trench 320 to reduce dark current noise associatedwith defects formed on the substrate upper surface 319 during formationof trench 320, for example during the etching of substrate upper surface319 to form trench 320. In embodiments, the passivation implant regionis formed by implanting p-type dopants (such as boron) for n-typephotodiode on the substrate upper surface 319 into the region betweenfloating diffusion region 360 and substrate upper surface 319. Inembodiments, the passivation implant region is formed by implantingn-type dopants (such as arsenic, phosphorus) for p-type photodiode onthe substrate upper surface 319 into the region between floatingdiffusion region 360 and substrate upper surface 319.

FIG. 4 is a cross-sectional view of a pixel 400, which is pixel 300 withthe addition of dielectric layer 450 that lines trench 320. Dielectriclayer 450 may include at least one of a nitride material and an oxidematerial. Dielectric layer 450 includes a thin section 451 betweenplanes 302 and 306, and a thick section 455 between planes 301 and 302.Thin section 451 and thick section 455 have respective widths 452 and456 in direction 298X, where width 456 is greater than or equal to width325, and width 452 is less than width 456. In embodiments, width 456 isthe sum of width 325 and width 452.

In embodiments, pixel 400 also includes a dielectric region 458 betweenplanes 301 and 302 and adjacent to floating diffusion region 360 suchthat floating diffusion region 360 is between dielectric region 458 anddielectric layer 450. Dielectric region 458 has width 456 and may beformed of the same material as dielectric layer 450.

In embodiments, pixel 400 includes a gate-electrode material 425 thatfills trench 320 between planes 301 and 306. Gate-electrode material 425may overfill trench 320 such that it extends above plane 301, as shownin FIG. 4. Gate-electrode material 425 may include at least one ofpolysilicon and a metal.

Distance 466 denotes a distance in direction 298X between floatingdiffusion region 360 and gate-electrode material 425 between planes 301and 303. Distance 466 is greater than or equal to width 456. Distance462 denotes a distance in direction 298X between floating diffusionregion 360 and gate-electrode material 425 between planes 303 and 306.Distance 462 is greater than or equal to width 452 and less thandistance 466. When thickness 312 equals zero, width 456 equals distance466 and width 452 equals distance 462.

In embodiments, width 456 is between 0.1 and 0.4 μm and, to accommodatethis width, upper width 322 may exceed lower width 327 by between 0.2 μmand 0.8 μm. In embodiments, shallow-trench depth 324 is greater than orequal to 0.1 μm; for example, shallow-trench depth 324 may be 0.25 μmand 0.30 μm. A technical benefit of these ranges of width 456 andshallow-trench depth 324, either individually or in combination, is tolower the capacitance of floating diffusion region 360 (junctioncapacitance), which increases pixel 400's conversation gain, and henceits sensitivity to small differences in photogenerated charges.

Trench 320, dielectric layer 450, and gate-electrode material 425collectively form a vertical transfer gate 480, which is an example ofvertical transfer gate 280, FIG. 2A Photodiode region 340, verticaltransfer gate 480, and floating diffusion region 360 operate as afield-effect transistor of pixel 400 that has a gate length 313. Gatelength 313 is a difference between photodiode depth 343 and junctiondepth 363. Appropriate choice of gate length 313 involves balancing atradeoff between pixel blooming—when gate length 313 is too long—andpunch-through between photodiode region 340 and floating diffusionregion 360 when gate length 313 is too short. In embodiments, gatelength 313 is between twenty nanometers and two hundred nanometers,which balances said tradeoff. In embodiments, width 452 is between twonanometers and ten nanometers such that the transfer transistorassociated with vertical transfer gate 480 has an appropriate thresholdvoltage.

It is appreciated by those skilled in the art the effective capacitanceof floating diffusion region 360 is related to conversion gain of theassociated pixel, e.g., pixel 300. The larger the effective capacitanceof floating diffusion region 360, the smaller the conversion gain of theassociated pixel, hence the lower the dynamic range that the pixel canprovide to the image signal output. The smaller the effectivecapacitance of floating diffusion region 360, the larger the conversiongain of the associated pixel, hence the higher the dynamic range thatthe pixel can provide to the image signal output.

Components that may contribute to the effective capacitance of floatingdiffusion region 360 include: (a) the junction capacitance betweenfloating diffusion region 360 and the semiconductor substrate 310, (b)the junction capacitance between floating diffusion region 360 andvertical transfer gate 480, (c) the parasitic capacitance betweenfloating diffusion region 360 and the gate of source follower transistorSF, (d) the parasitic capacitance between floating diffusion region 360and the drain of reset transistor RST, and (e) the coupling capacitancebetween the metal contact connecting floating diffusion region 360 andthe nearby metal contact (e.g., metal connect connecting to verticaltransfer gate 480), wherein junction capacitances contribute the most tothe overall effective capacitance of floating diffusion region 360. Ashallow trench isolation structure with dielectric layer 450 separatingfloating diffusion region 360 and vertical transfer gate 480 caneliminate the junction capacitance between floating diffusion region 360and vertical transfer gate 480, thus reducing the effective capacitanceof floating diffusion region 360.

Dielectric region 458 also further lowers the junction capacitance byreducing the junction area between floating diffusion region 360 (e.g.,n-type doped region) and surrounding semiconductor substrate 310 (e.g.,p-type doped substrate).

In embodiments, lower photodiode section 341 photogenerates andaccumulates photoelectrons and upper photodiode section 345 functions totransfer charges acuminated in the lower photodiode section 341 to aconduction channel formed by vertical transfer gate 480 of the transfertransistor when the transfer transistor is biased to turn on duringchange transfer operation.

In embodiments, floating diffusion region 360 and upper photodiodesection 345 are on the same side of trench 320, as illustrated in FIG.3. In such embodiments, part of floating diffusion region 360 isdirectly between the upper photodiode section 345 and thick section 455,which enables gate length 313 to be smaller than when floating diffusionregion 360 and upper photodiode section 345 are on opposite sides oftrench 320

In embodiments, trench depth 323 is between 0.2 μm and 0.7 μm. Whentrench depth 323 is too large, e.g., more than 0.7 μm, the bottom oftrench 320 is too close to lower photodiode section 341, which resultsin increased white-pixel artifacts (from dark current) and reducedfull-well capacity of photodiode region 340. When trench depth 323 istoo small, e.g., less than 0.2 μm, shallow-trench depth 324 is toosmall, as described above, and the upper limit of gate length 313 is toofar constrained such that vertical transfer gate 480 does not functionproperly.

FIG. 5 is a schematic cross-sectional view of part of an image sensor590, which includes two adjacent pixels 500(1) and 500(2) in asemiconductor substrate 510. Semiconductor substrate 510 is an exampleof semiconductor substrate 310, and includes planar region 518, surface519, and trenches 520(1, 2), which are examples planar region 318,surface 319, and trench 320, respectively. Pixels 300 and 400 are eachexamples of pixel 500. For example, when pixels 500 include a respectivedielectric layer 450, each pixel 500 is equivalent to a respective pixel400, and each may include at least one of dielectric layer 450 andgate-electrode material 425 in its trench 520.

Semiconductor substrate 510 includes a common floating diffusion 560between trenches 520(1, 2). Common floating diffusion 560 functions as afloating diffusion region shared by both pixels 500(1) and 500(2). Eachpixel 500 includes a respective photodiode region 340. Thickness 312separates floating diffusion region 560 from surface 519 of trenches520. As previously noted, thickness 312 may equal zero, in which case,floating diffusion region 560 spans semiconductor substrate 510 betweentrenches 520(1) and 520(2).

In the example illustrated by FIG. 5, sides of the common floatingdiffusion 560 are surrounded by dielectric layer 450, which reduces thejunction area between common floating diffusion 560 and surroundingsemiconductor substrate 510, thus minimizing the junction capacitanceassociated with common floating diffusion 560 to the junctioncapacitance formed between the bottom area of the common floatingdiffusion 560 and the semiconductor substrate 510. It should beappreciated that the effective capacitance of common floating diffusion560 can be modulated by configuring the bottom width of the commonfloating diffusion 560 and the depth of shallow trench depth of trench520(1, 2).

Excess photoelectrons generated by each respective photodiode region 340during an integration period of image sensor 590 in the depicted pixelstructure would travel to the common floating diffusion 560 through theanti-blooming path formed between each of the photodiode sections 340and the common floating diffusion 560, instead to the nearby photodioderegion 340 of adjacent pixel affecting the sensitivity of adjacent pixelas the effective distance between each respective photodiode region 340and the common floating diffusion 560 is shorter than the distance tothe nearby photodiode region 340 of adjacent pixel, as such bloomingissue can be improved.

FIG. 6 is a cross-sectional view of an image sensor 690, which is anexample of image sensor 590. FIG. 5 is a cross-sectional view of imagesensor 690 in either cross-sectional plane 5A or 5B shown in FIG. 6.Image sensor 690 includes pixels 600(1-4), each of which is an exampleof pixel 500. Image sensor also includes dielectric layers 650 and acommon floating diffusion region 660, which are respective examples ofdielectric layers 450 and common floating diffusion region 560. Thecross-sectional view of FIG. 6 is in a cross-sectional plane 6 shown inFIG. 5, and is parallel to the x-y plane.

FIG. 7 is a cross-sectional view of an image sensor 790, which is anexample of image sensor 590. FIG. 5 is a cross-sectional view of imagesensor 790 in cross-sectional plane 5C shown in FIG. 7. Image sensor 790includes pixels 700(1-4), each of which is an example of pixel 500.Image sensor also includes a dielectric layer 750 and a common floatingdiffusion region 760, which are respective examples of dielectric layers450 and common floating diffusion region 560. In image sensor 790,respective dielectric layers (e.g., layers 450 and 550) of adjacentpixels are connected to form dielectric layer 750 that surrounds commonfloating diffusion region 760. Dielectric layer 750 further lowers thejunction capacitance of floating diffusion region 360 relative todielectric layers 650. The cross-sectional view of FIG. 7 is in across-sectional plane 7 shown in FIG. 5, and is parallel to the x-yplane.

FIG. 8 is a cross-sectional schematic of a semiconductor substrate 810that includes a shallow trench 820 and a shallow trench 828, which arefilled with a dielectric 855 and a dielectric 858, respectively. Inembodiment, semiconductor substrate 810 is photo-resist patterned andetched to form shallow trench 820 and shallow trench 828 in thesemiconductor substrate 810. Subsequently, dielectric material isdeposited filling shallow trench 820 and shallow trench 828.Semiconductor substrate 810 is an example of semiconductor substrate310. Dielectric 855 is an example of thick section 455 of dielectriclayer 450. Dielectric 858 in shallow trench 828 is similar to dielectricregion 458.

FIG. 9 is a cross-sectional schematic of a semiconductor substrate 910,which is semiconductor substrate 810 after formation of a floatingdiffusion region 960 therein. FIG. 9 depicts a patterned hard mask 902on semiconductor substrate 910 that defines transverse boundaries offloating diffusion region 960 during implantation thereof. Floatingdiffusion region 960 is an example of floating diffusion region 360,FIG. 3.

FIG. 10 is a cross-sectional schematic of a semiconductor substrate1010, which is semiconductor substrate 910 after formation of a deeptrench 1020 therein. Semiconductor substrate 1010 includes a thickdielectric section 1055, and the remainder of dielectric 855 afterformation of deep trench 1020. Each dielectric section 1055 is anexample of thick section 455, FIG. 4. FIG. 10 depicts a patterned hardmask 1002 on semiconductor substrate 1010 that defines transverseboundaries of deep trench 1020 during etching thereof. Deep trench 1020may be formed by first etching dielectric 855 not covered by hard mask1002, and then by etching semiconductor substrate 1010 therebeneath. Inembodiments, semiconductor substrate 1010 includes a gate-oxide layer1150 that lines deep trench 1020. Deep trench 1020 extends intosemiconductor substrate 1010; part of deep trench 1020 is adjacent toupper photodiode section 345.

FIG. 11 is a cross-sectional schematic of a pixel 1100, which issemiconductor substrate 1010 after filling deep trench 1020 withgate-electrode material 425. Pixel 1100 is an example of pixel 400.

FIG. 12 is a flowchart illustrating a method 1200 for fabricating apixel, such as pixel 300. Method 1200 includes steps 1210 and 1240. Inembodiments, method 1200 also includes at least one of steps 1220, 1230,1250, and 1260.

Step 1210 includes forming a wide trench in a semiconductor substratehaving an upper depth with respect to a planar top surface of thesemiconductor substrate. In an example of step 1210, wide trench 321 isformed in semiconductor substrate 310, via an etching process forexample.

Step 1220 includes forming a photodiode region in the semiconductorsubstrate. In embodiments, step 1220 follows step 1210, such that thephotodiode region is formed after the formation of the wide trench. Thephotodiode region may be formed via ion implantation. In an example ofstep 1220, photodiode region 340 is formed in semiconductor substrate310.

Step 1230 includes filling the wide trench with a dielectric material.In an example of step 1230, wide trench 321 is filled with the materialthat forms dielectric layer 450. Step 1230 may be executed before orafter formation of narrow trench 326 and/floating diffusion region 360in semiconductor substrate 310.

Step 1240 includes ion-implanting, between the planar top surface and ajunction depth in the semiconductor substrate, a floating-diffusionregion. The floating diffusion region has, in a cross-sectional planeperpendicular to the planar top surface, (i) an upper width between theplanar top surface and the upper depth, and (ii) between the upper depthand the junction depth, a lower width that exceeds the upper width. Partof the floating-diffusion region is beneath the wide trench and betweenthe upper depth and the junction depth. In an example of step 1240,floating diffusion region 360 is ion-implanted between planar region 318and junction depth 363. In embodiments, step 1240 includes masking thesemiconductor substrate with a patterned mask, such as patterned mask902, FIG. 9.

In embodiments, step 1240 precedes at least one of steps 1210 and 1230.In embodiments, step 1240 follows at least one of steps 1210 and 1230.An advantage of step 1210 preceding step 1240 is that the dopedsemiconductor in the floating diffusion region has different etch ratethan undoped parts of the semiconductor substrate. Hence, etching thewide trench after the ion-implantation of step 1240 is more difficultthan etching before the ion-implantation of step 1240.

Step 1250 includes forming, within the wide trench, a narrow trench inthe semiconductor substrate that extends from the upper depth to atrench depth that exceeds the junction depth. In the cross-sectionalplane, the narrow trench has a width that is less than a width of thewide trench. In embodiments, step 1250 follows step 1230, such that partof the dielectric material filling the wide trench is removed, e.g. viadry etching, while forming the narrow trench. In an example of step1250, narrow trench 326 is formed within wide trench 321.

Step 1260 includes lining the narrow trench with a dielectric layer, athickness of the dielectric layer between the planar top surface and theupper depth exceeding a thickness of the dielectric layer between theupper depth and the trench depth. In an example of step 1260, narrowtrench 326 is lined with dielectric layer 450 between planes 302 and306, which yields pixel 400. In embodiments, narrow trench 326 isfurther deposited with conductive material, to form a vertical transfergate e.g., vertical transfer gate 480.

Combinations of Features

Features described above as well as those claimed below may be combinedin various ways without departing from the scope hereof. The followingenumerated examples illustrate some possible, non-limiting combinations:

(A1) A pixel includes a semiconductor substrate, a floating diffusionregion, and a photodiode region. The semiconductor substrate has asubstrate upper surface forming a trench extending into thesemiconductor substrate. The trench has a (i) trench depth relative to aplanar region of the substrate upper surface surrounding the trench, andin a cross-sectional plane perpendicular to the planar region and in adirection parallel to the planar region (ii) an upper width between theplanar region and an upper depth that is less than the trench depth, and(iii) a lower width, between the upper depth and the trench depth, thatis less than the upper width. The floating diffusion region is in thesemiconductor substrate, adjacent to the trench and extends away fromthe planar region to a junction depth, relative to the planar region,that exceeds the upper depth and is less than the trench depth. Thephotodiode region is in the semiconductor substrate and includes (i) alower photodiode section beneath the trench and (ii) an upper photodiodesection adjacent to the trench, beginning at a photodiode depth that isless than the trench depth, extending toward and adjoining the lowerphotodiode section.

(A2) In pixel (A1), in the cross-sectional plane, the floating diffusionregion may have (i) an upper width between the planar region and theupper depth, and (ii) between the upper depth and the junction depth, alower width that exceeds the upper width.

(A3) Any of pixels (A1) and (A2) may further include, when the upperwidth exceeds the lower width by a distance 2ΔW in the cross-sectionalplane, a dielectric layer lining the trench. In the cross-sectionalplane in a transverse direction parallel to the planar region, thedielectric layer has (i) a thick section between the upper depth and theplanar region having an upper width W_(u) greater than or equal to ΔW,and (ii) a thin section between the upper depth and the trench depth andhaving a lower width W_(l)<W_(u). In the cross-sectional plane, part ofthe floating diffusion region is directly between the upper photodiodesection and the thick section.

(A4) In pixel (A3), the upper width W_(u) may be equal to (W₁+ΔW).

(A5) Any of pixels (A3) and (A4) may further include a dielectric regionextending from the substrate upper surface to the junction depth andadjacent to the floating diffusion region, in which the floatingdiffusion region is between the trench and the dielectric region.

(A6) In any of pixels (A3)-(A5), in an additional cross-sectional planeparallel to the planar region and between the planar region and theupper depth, the dielectric layer may surround the trench.

(A7) Any of pixels (A3)-(A6) may further include a gate-electrodematerial filling the trench; the trench, the dielectric layer, and thegate-electrode material forming a vertical transfer gate electricallyconnected to the photodiode region.

(A8) In pixel (A7) between the planar region and the upper depth, adistance d_(u) between the floating diffusion region and thegate-electrode material may be greater than or equal to upper widthW_(u), and between the upper depth and the trench depth, a distanced_(l) between the floating diffusion region and the gate-electrodematerial may satisfy W_(l)≤d_(l)<W_(u) and d_(l)<d_(u).

(A9) In any of pixels (A1)-(A8), the upper width may exceed the lowerwidth by between 0.2 μm and 0.8 μm.

(A10) In any of pixels (A1)-(A9), the trench depth may be between 0.2 μmand 0.7 μm.

(A11) In any of pixels (A1)-(A10), the junction depth may exceed theupper depth by between ten nanometers and one hundred nanometers.

(A12) In any of pixels (A1)-(A11), the photodiode depth may exceed thejunction depth by between twenty nanometers and two hundred nanometers.

(A13) In any of pixels (A1)-(A12), the upper depth may be greater thanor equal to 0.1 μm.

(B1) An image sensor includes a first instance of any of the pixel(A1)-(A13); and a second instance of any of the pixel (A1)-(A13) in thesemiconductor substrate of, and adjacent to, the first instance. Thefloating diffusion region of the first instance and the floatingdiffusion region of the second instance are part of a common floatingdiffusion region, at least part of which is between the first instanceand the second instance.

(C1) An image sensor includes a first instance of any of the pixel(A3)-(A13); and a second instance of any of the pixel (A3)-(A13) in thesemiconductor substrate of, and adjacent to, the first instance. Thefloating diffusion region of the first instance and the floatingdiffusion region of the second instance are part of a common floatingdiffusion region, at least part of which is between the first instanceand the second instance. The common floating diffusion region spans aregion of the semiconductor substrate between the respective dielectriclayers between the first instance and the second instance.

(C2) In pixel (C1), the dielectric layer of the first instance and thedielectric layer of the second instance may be part of a common adielectric layer, at least part of which is between the trench of thefirst instance and the trench of the second instance.

(D1) A method for forming a pixel includes forming a wide trench in asemiconductor substrate and ion-implanting a floating-diffusion regionin the semiconductor substrate between the planar top surface and ajunction depth in the semiconductor substrate. The wide trench has anupper depth with respect to a planar top surface of the semiconductorsubstrate. The floating-diffusion region has, in a cross-sectional planeperpendicular to the planar top surface, (i) an upper width between theplanar top surface and the upper depth, and (ii) between the upper depthand the junction depth, a lower width that exceeds the upper width. Partof the floating-diffusion region is beneath the wide trench and betweenthe upper depth and the junction depth.

(D2) Method (D1) may further include filling the wide trench with adielectric material.

(D3) Any one of methods (D1) and (D2) may further include forming,within the wide trench, a narrow trench in the semiconductor substratethat extends from the upper depth to a trench depth that exceeds thejunction depth. In the cross-sectional plane, the narrow trench having awidth that is less than a width of the wide trench.

(D4) Method (D3) may further include lining the narrow trench with adielectric layer, a thickness of the dielectric layer between the planartop surface and the upper depth exceeding a thickness of the dielectriclayer between the upper depth and the trench depth.

Changes may be made in the above methods and systems without departingfrom the scope of the present embodiments. It should thus be noted thatthe matter contained in the above description or shown in theaccompanying drawings should be interpreted as illustrative and not in alimiting sense. Herein, and unless otherwise indicated the phrase “inembodiments” is equivalent to the phrase “in certain embodiments,” anddoes not refer to all embodiments. The following claims are intended tocover all generic and specific features described herein, as well as allstatements of the scope of the present method and system, which, as amatter of language, might be said to fall therebetween.

What is claimed is:
 1. A method for forming a pixel comprising: forming,in a semiconductor substrate, a wide trench having an upper depth withrespect to a planar top surface of the semiconductor substrate; andion-implanting, between the planar top surface and a junction depth inthe semiconductor substrate, a floating-diffusion region having, in across-sectional plane perpendicular to the planar top surface, (i) anupper width between the planar top surface and the upper depth, and (ii)between the upper depth and the junction depth, a lower width thatexceeds the upper width, part of the floating-diffusion region beingbeneath the wide trench and between the upper depth and the junctiondepth.
 2. The method of claim 1, said forming the wide trench precedingsaid ion-implanting the floating-diffusion region.
 3. The method ofclaim 1, further comprising filling the wide trench with a dielectricmaterial.
 4. The method of claim 3, said ion-implanting thefloating-diffusion region preceding said filling the wide trench.
 5. Themethod of claim 1, further comprising forming, within the wide trench, anarrow trench in the semiconductor substrate that extends from the upperdepth to a trench depth that exceeds the junction depth, in thecross-sectional plane, the narrow trench having a narrow-trench widththat is less than a wide-trench width of the wide trench.
 6. The methodof claim 5, further comprising lining the wide trench and the narrowtrench with a dielectric layer, in a direction parallel to the planartop surface, an upper-thickness of the dielectric layer between theplanar top surface and the upper depth exceeding a lower-thickness ofthe dielectric layer between the upper depth and the trench depth. 7.The method of claim 5, further comprising forming a vertical transfergate by depositing a conductive material in the narrow trench.
 8. Themethod of claim 5, further comprising, before forming the narrow trench,filling the wide trench with a dielectric material.
 9. The method ofclaim 8, forming the narrow trench comprising: removing a volume of thedielectric material to yield an inner-surface of the dielectric materialthat defines an aperture through the dielectric material.
 10. The methodof claim 9, further comprising lining the inner-surface and the narrowtrench with an oxide layer.
 11. The method of claim 1, furthercomprising forming a photodiode region in the semiconductor substrate.12. The method of claim 11, said forming the photodiode region occurringafter said forming the wide trench, and said forming the photodioderegion comprising forming the photodiode region at least partiallybeneath the wide trench.
 13. The method of claim 11, in thecross-sectional plane, the floating-diffusion region being on a firstside of the wide trench, said forming the photodiode region comprisingforming part of the photodiode region on the first side of the widetrench.
 14. The method of claim 11, said forming the photodiode regionoccurring before said ion-implanting of the floating-diffusion region.15. The method of claim 11, forming the photodiode region comprisingion-implanting the photodiode region.
 16. The method of claim 15, thesemiconductor substrate having a bottom surface opposite the planar topsurface, ion-implanting the photodiode region comprising ion implantingthe photodiode region to a depth, with respect to the bottom surface,that is less than a distance between the bottom surface and the junctiondepth.
 17. The method of claim 11, further comprising: forming, in thesemiconductor substrate, an additional wide trench having the upperdepth with respect to the planar top surface of the semiconductorsubstrate; and forming an additional photodiode region in thesemiconductor substrate and at least partially beneath the additionalwide trench.
 18. The method of claim 17, forming the additional widetrench preceding said ion-implanting of the floating-diffusion region,in which an additional part of the floating-diffusion region is beneaththe additional wide trench and between the upper depth and the junctiondepth.
 19. The method of claim 11, further comprising: forming, in thesemiconductor substrate, a first, a second, and a third additional widetrench, each additional wide trench having the upper depth with respectto the planar top surface of the semiconductor substrate; and forming,in the semiconductor substrate and beneath the first, a second, and athird additional wide trench, a respective first, second, and thirdadditional photodiode region.